Key Points MicroRNA (miRNA) is a single-stranded RNA of ∼22 nucleotides in length, which is generated by an RNase-III-type enzyme from an endogenous transcript that contains a local hairpin structure. miRNA functions as a guide molecule in post-transcriptional gene silencing, by base pairing with the target mRNAs, which leads to mRNA cleavage or translational repression. By silencing various target mRNAs, miRNAs have key roles in diverse regulatory pathways, including control of development timing, haematopoietic cell differentiation, apoptosis, cell proliferation and organ development. miRNA genes belong to class II genes, which are transcribed by RNA polymerase II. A majority of miRNA loci are found in intronic regions of protein-coding or non-coding transcription units, whereas the others are found in exonic regions of non-coding transcription units. In animals, miRNA genes are transcribed to generate long primary transcripts (pri-miRNAs), which are first cropped by RNase-III-type enzyme Drosha to release the hairpin intermediates (pre-miRNAs) in the nucleus. Drosha forms a large (500–650 kDa) complex, known as the Microprocessor complex, together with its essential cofactor DGCR8/Pasha, which contains two dsRNA-binding domains. Pre-miRNA then gets exported to the cytoplasm by exportin-5, which is a member of the Ran-dependent nuclear transport receptor family. Following arrival in the cytoplasm, pre-miRNAs are subjected to the second processing step, which is carried out by Dicer, the cytoplasmic RNase-III-type protein. In plants that lack Drosha and DGCR8, it has been suggested that miRNA processing is executed by Dicer-like protein 1 (DCL1) in the nucleus and that nuclear export is mediated by HASTY, the exportin-5 homologue.
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